Definitely sounds like compost issues to me Jacqui.
I sowed my tomatoes on April 7th this year and my Sungold and Zenith had been transplanted twice by day 19 as they grew so fast. It was a very sunny April here, but I just potted on into local garden centre own-brand MPC with a smidgen of rock dust, fungi and friendly bugs and all nine strains grew like the wind. They had to be held back indoors for a few days in early May when the nights were 3-5C, but they were in final pots this year after 5-6 weeks.
If you have space inside to keep 12-18 8cm pots on a sunny windowsill, growth in the first four weeks can be very rapid indeed with later sowings. I have experimented sowing from early Feb to late April and the only reason to sow early in my experience is to deliberately get a crop in mid to late June. If you intend putting into final positions in mid May, plants will be plenty big enough sown in early April if they mostly live indoors.
Obviously if they live in a greenhouse, things may be a bit different.